The present invention generally relates to three position solenoid controlled valves and more particularly, to a three position solenoid controlled valve suitably used for a brake fluid pressure control device such as an anti-skid brake control device.
Generally, in antiskid control, a pressurizing mode, a holding mode and a depressurizing mode in which liquid pressure of wheel cylinders for wheels of a motor vehicle is increased, held and reduced, respectively are changed over in accordance with skid state of the wheels so as to control liquid pressure of the wheel cylinders.
In order to produce the above mentioned pressurizing mode, the holding mode and the depressurizing mode, a structure is proposed in which an inlet valve formed by a two port and two position solenoid controlled selector valve is provided between a master cylinder of the motor vehicle and each of the wheel cylinders and a discharge valve formed by a two port and two position solenoid controlled selector valve is provided in a return path returning from each of the wheel cylinders to the master cylinder through a reservoir, etc.
On the other hand, a single three position solenoid controlled valve capable of producing these three modes is known. For example, such a single three position solenoid controlled valve is proposed in Japanese Patent Laid-Open Publication No. 49-83028 (1974). In this known three position solenoid controlled valve, and as illustrated in FIG. 1, a slider 2 is slidably provided in a liquid chamber 1d of a housing 1 having first, second and third ports 1a, 1b and 1c. Meanwhile, an electromagnet 3 is mounted on the housing 1. Valve bodies 4A and 4B are provided in the liquid chamber 1d so as to be movable relative to the slider 2. while a second spring 6 is compressed between the valve bodies 4A and 4B. A stopper 7 is slidable relative to the slider 2 and a third spring 8 is compressed between the stopper 7 and the slider 2.
When this three position solenoid controlled valve is used for antiskid control, the first port 1a is connected with the master cylinder, the second port 1b is connected with each of the wheel cylinders and the third port 1c is connected with the return path. Meanwhile, by changing amount of electric power supplied to the electromagnet 3 so as to control urging force (electromagnetic force) applied to the slider 2 by the electromagnet 3, the pressurizing mode, the holding mode and the depressurizing mode are produced.
At the time of deenergization of the electromagnet 3 as shown in FIG. 1, since urging force of the first spring 5 exceeds that of the second spring 6, the valve body 4B opens the first port 1a, while the valve body 4A closes the third port 1c. At this time, the first and second ports 1a and 1b are communicated with each other, thereby resulting in the pressurizing mode of antiskid control.
On the other hand, when the electromagnet 3 is energized so as to apply to the slider 2 a electromagnetic force which is larger than a difference between an urging force of the first spring 5 and that of the second spring 6 but is smaller than a sum of urging forces of the first, second and third springs 5, 6 and 8, the slider 2 is displaced leftwards in FIG. 1, while the valve body 4B closes the first port 1a with a force equal to a difference between a sum of a electromagnetic force of the electromagnet 3 and a leftward urging force of the second spring 6 and a rightward urging force of the first spring 5. Meanwhile, the third port 1c is kept closed by the valve body 4A with a force equal to the urging force of the second spring 6. In this state, the second port 1b is communicated with none of the first and third ports 1a and 1c, thus resulting in the holding mode of antiskid control.
When electromagnetic force of the electromagnet 3 is increased by increasing amount of electric power supplied to the electromagnet 3, the slider 2 overcomes the urging forces of the first, second and third springs 5, 6 and 8 and is further displaced leftwards in FIG. 1. Thus, the valve body 4A opens the third port 1c, while the valve body 4B closes the first port 1a. In this state, the second port 1b is communicated with the third port 1c, thereby resulting in the depressurizing mode of antiskid control.
Electromagnetic force required for opening and closing the first and the third ports 1a and 1c by the valve bodies 4A and 4B through displacement of the slider 2 as described above is associated with the first, second and third springs 5, 6 and 8 but should be actually determined also in view of liquid pressure applied to the first and third ports 1a and 1c.
Initially, in the holding mode, since liquid pressure from the master cylinder is applied to the valve body 4A closing the third port 1c, an electromagnetic force which is equal to a sum of the difference be:ween the urging force of the first spring 5 and that of the second spring 6 and a product of a maximum pressure of the master cylinder and a sealing area of the valve body 4A is required to be applied to the slider 2. Therefore, large electromagnetic force is necessary for maintaining the holding mode.
Meanwhile, in the holding mode, urging force of the third spring 8 should be set relatively large such that liquid pressure of the master cylinder does not cause the valve body 4B to erroneously open the first port 1a. In the depressurizing mode, electromagnetic force exceeding the sum of the urging forces of the first, second and third springs 5, 6 and 8 is necessary as described above. If the urging force of the third spring 8 is set large, electromagnetic force required for producing the depressurizing mode also becomes relatively large.
Thus, in the known three position solenoid controlled valve shown in FIG.1, since electromagnetic force required for producing the holding mode and the depressurizing mode is relatively large, the electromagnet should be made large in size so as to obtain large electromagnetic force, thereby resulting in increase of size and rise of production cost of the solenoid controlled valve as a whole.
Meanwhile, in the holding mode, electromagnetic force of the electromagnet should fall within a predetermined range. However, as shown in FIG. 2, electromagnetic force P of general electromagnets is substantially proportional to square of electric current I. Therefore, a range .increment.I.sub.2 of electric current I, which corresponds to a predetermined range .increment.P of electromagnetic force P when electromagnetic force P is large, becomes considerably small in comparison with a range .increment.I.sub.1 of electric current I corresponding to the predetermined range .increment.P of electromagnetic force P when electromagnetic force P is small. Therefore, in order to produce the holding mode in the known three position solenoid controlled valve of FIG. 1, electric current should be controlled highly accurately.
Meanwhile, in the known three position solenoid controlled valve, since magnitude of electromagnetic force is influenced by various factors such as position of the movable members in addition to magnitude of electric current, an electric current controller, components of solenoid controlled valve, etc. are required to be produced with high dimensional precision.
In order to solve these problems of the known three position solenoid controlled valve, a directional control valve in which force produced by difference in liquid pressure, i.e., force based on liquid pressure is not applied by using a spool is proposed as shown in Fig. 3. In this prior art directional control valve, a spool 12 is slidably provided in a liquid chamber 11d of a housing 11 having first, second and third ports 11a, 11b and 11c. A liquid passage 12a extends axially through the spool 12 and the spool 12 is formed with passages 12b and 12c for communicating the fluid passage 12a with surface of the spool 12. A spring 13 is compressed between the spool 12 and the liquid chamber 11d so as to urge the spool 12 leftwards in FIG. 3 and an electromagnet 14 is provided in the housing 11. In case this prior art directional control valve is used for antiskid control, the first, second and third ports 11a, 11b and 11c are, respectively, connected with the master cylinder, the return path and each of the wheel cylinders.
At the time of deenergization of the electromagnet 14 as shown in FIG. 3, the first and third ports 11a and 11c are communicated with each other via the liquid passage 12a and the passages 12b and 12c, thereby resulting in the pressurizing mode of antiskid control. At this time, since liquid pressure of the master cylinder is applied to opposite ends 12d and 12e of the spool 12, force based on liquid pressure is not produced.
When the electromagnet 14 is energized so as to displace the spool 12 rightwards in FIG. 3 against urging force of the spring 13, communication between the first port 11a and the passage 12b is cut off and the third ports 11c is communicated with none of the first and second ports 11a and 11b, thus resulting in the holding mode of antiskid control.
Furthermore, when amount of electric power supplied to the electromagnet 14 is increased so as to displace the spool 12 rightwards in FIG. 3, the passage 12b is communicated with the second port 11b and the second port 11b is communicated with the third port 11c by way of the liquid passage 12a and the passages 12b and 12c, thereby resulting in the depressurizing mode of antiskid control.
In both of the holding mode and the depressurizing mode, liquid pressures applied to the opposite ends 12d and 12e of the spool 12 are identical with each other and thus, no force based on liquid pressure is applied to the spool 12. Therefore, in this prior art directional control valve, electromagnetic force required for producing the holding mode and the depressurizing mode is made smaller than that of the known three position solenoid controlled valve of FIG. 1.
However, since working fluid leaks through a sliding surface S of the spool 12, this prior art directional control valve is not suitable for an antiskid control device in which even slight leakage of working fluid during inoperative period is not permissible.
Meanwhile, in order to solve this problem of prior art directional control valve, Japanese Patent Laid-Open Publication No. 57-104446 (1982) proposes an antiskid control device including a solenoid controlled valve in which axial force based on liquid pressure is balanced using a sliding seal. However, in this conventional arrangement employing the sliding seal, since accuracy of pressure balance is not sufficiently high and sliding resistance is produced, it is difficult to put the known antiskid control device to practical use.